The present invention relates to an internal combustion engine having an exhaust-driven turbocharger for raising the intake manifold pressure of the air and fuel mixture, together with a device for limiting the manifold boost pressure generated by the turbocharger.
An engine having a turbocharger of this type is described in Society of Automotive Engineers Technical Paper 780413 entitled "Buick's Turbocharged V-6 Power Train for 1978". Since the turbocharger speed is proportional to the volume of exhaust gases forced through the exhaust turbine of the turbocharger, the boost pressure generated by the turbocharger compressor in the intake manifold increases with engine RPM. In the Buick engine, however, when the boost pressure in the manifold reaches (at increasing RPM) a pre-selected maximum, a pressure sensor in the manifold opens a wastegate (exhaust by-pass valve) upstream of the turbocharger to divert some of the exhaust gas from the exhaust turbine. Thus the boost pressure will not continue to increase with increased engine RPM, but will remain at the pre-selected maximum. The engine also has a detonation sensor which retards the ignition timing when engine knocking is detected.
In the Buick engine, the adjustment of the ignition timing is done as an emergency measure since the wastegate limits the turbocharger boost pressure to a value below that at which knocking ordinarily occurs. When knocking occurs, however, and the ignition timing is thus retarded, the exhaust gas temperature increases which may result in damage to the turbine, already greatly thermally and mechanically loaded.
Since the amount of fuel consumed in an engine of this type remains constant at the retardation of ignition timing and the power output of the engine decreases with decreased boost pressure, the optimal efficiency of the engine occurs at the boost pressure at which knocking occurs. In the Buick engine, however, this value of the manifold boost pressure cannot be adjusted and thus the engine operates at less than optimum efficiency.